The present invention relates to devices (principally lamps) to evaluate the stability or persistence of the color match of similarly colored objects. The device of this invention provides for detection of potential mismatches in colors which appear to match under some illuminants but may not match under other illuminants.
A large number of commercial products owe their customer-acceptance partly to the fact that they match, in color and lightness, some other product or some other part of the same product. Automobile upholstery and body paint are one example. This match should persist acceptably under whatever illuminant the customer may view the product.
Obtaining an initial color match (under daylight, for example) is a difficult and complex industrial problem in the common case where the matching parts are colored by different pigments or consist of different materials. Even after the initial color match under daylight has been achieved, however, a potential mismatch still remains when the products are viewed under different illuminants. A manufacturer may have the product inspected under one or two additional illuminants such as an incandescent lamp or a fluorescent lamp. Considerable effort can be extended in adjusting pigment and dye formations until a satisfactory match persists under all test illuminants. However this still does not eliminate all of the possible mismatches. The colors of the automotive upholstery and paint may be viewed not only under daylight, incandescent lamps, and different types of fluorescent lamps, but also under other lamps such as high pressure sodium lamps, metal halide lamps, and both corrected and uncorrected mercury lamps. If the spectral reflectance curves of the materials are identical, the color match will persist under all illuminants. This, however, is generally not the case and it is generally impractical to make the spectral reflectance curves identical. Thus the manufacturer generally must test for a color match under a large number of lamps and make repeated corrections if he wishes to be sure that the color match will persist under most different illuminants. Even then, it is possible that some other lamp will cause a mismatch.
FIG. 1 shows spectral reflectance curves measured from two yellow materials. While these materials were found by a normal human observer to match in color and lightness when illuminated by average daylight, it can be seen that these spectral reflectance curves are significantly different. FIG. 2 shows the spectral power distributions of the lights reflected (and thus the lights which would enter the eye) from the materials of FIG. 1 when illuminated by average daylight. The normal human eye perceives the two materials as having the same lightness and color despite the fact these spectral power distributions of the lights entering the eye from the two materials are significantly different.
FIG. 3 shows the spectral reflectance curves of two pinkish grey materials. These materials were also found to match in color and lightness when illuminated by average daylight. These materials are more strongly metameric than the materials of FIG. 1; i.e., the potential mismatch under other illuminatns is greater because of the large reflectance discrepancies. The reflectance differences (the areas, in the visible region, between the two curves) determine what is called the degree of metamerism. The degree of metamerism is generally a measure of the differences in color and/or brightness between the lights reflected from a pair of objects as the objects are illuminated by various illuminants. The larger the reflectance differences, the larger the possible mismatch. If there is no area between the loops, that is if the two reflectance curves are identical and coincident, the two objects will appear to match under any illuminant.